Mechanical ventilation in the presence of sleep disordered breathing

ABSTRACT

A method for controlling operation of a CPAP apparatus. The apparatus has a blower ( 2 ), a patient interface ( 6 ), an air delivery conduit ( 8 ) for delivering air from the blower ( 2 ) to the patient interface ( 6 ), a sensor ( 4   p ) for determining the pressure in the patient interface ( 6 ), and a control mechanism ( 15 ) that causes air to be delivered at a desired pressure to the patient interface ( 6 ) and that detects transitions between inhalation and exhalation of a respiratory cycle of a patient in order to synchronise the blower output with the patient&#39;s efforts. In one form the CPAP apparatus provides pressure in accordance with a bi-level waveform with at least one characterising parameter of the waveform being automatically adjusted in accordance with indications of sleep disordered breathing. The indications of sleep disordered breathing can be one or more of snoring, apnea, hypopnea, and flow limitation.

This application claims the priority of U.S. provisional application No.60/533,411 filed on Dec. 29, 2003.

1. FIELD OF THE INVENTION

This invention relates to mechanical ventilation of sleep disorderedbreathing (SDB).

2. BACKGROUND OF THE INVENTION

A comprehensive background discussion of mechanical ventilation can befound in “Principles and Practice of Mechanical Ventilation” (1994)Edited by Martin J Tobin, published by McGraw-Hill Inc., ISBN0-07-064943-7.

The use of nasal Continuous Positive Airway Pressure (CPAP) to treatObstructive Sleep Apnea (OSA) was invented by Colin Sullivan, see U.S.Pat. No. 4,944,310. Generally, the treatment involves providing a supplyof air or breathable gas from a blower to a patient via an air deliveryconduit and a patient interface. While treatment is effective, somepatients find it uncomfortable. Improving patient comfort and complianceis a continuing challenge.

One way to improve comfort is to provide a more comfortable patientinterface. In this regard, the ResMed MIRAGE™ masks have providedsignificant improvement in comfort. See U.S. Pat. Nos. 6,112,746;6,357,441; 6,581,602 and 6,634,358. A more recent development is theResMed MIRAGE™ ACTIVA™ mask series. See International Patent ApplicationWO 2001/97893.

In the early days of nasal CPAP systems for treating OSA, patients werefirst titrated in a clinical study to determine an optimal treatmentpressure. Titration involves a patient sleeping overnight in a clinicand being tested with a mask and CPAP device. The treatment pressureprovided by the CPAP device is adjusted until apneas are eliminated. Thetreatment pressure is usually in the range of 4-20 cmH₂O. A device wouldbe set to that pressure and given to the patient to take home. Asubsequent development was the automatically adjusting device that apatient could take home. The automatically adjusting device will raiseand/or lower the treatment pressure based on indications of obstructivesleep apnea, such as snoring. Such devices are sometime genericallyreferred to as Automatic Positive Airway Pressure (APAP) devices. SeeU.S. Pat. Nos. 5,245,995; 6,398,739; and 6,635,021.

Another type of nasal CPAP device provides a first pressure duringinhalation (sometimes termed an IPAP) and a second, lower pressureduring exhalation (sometimes termed and EPAP). Examples of these includethe ResMed VPAP™ series, and the Respironics BiPAP series. Bilevel CPAPdevices may be prescribed for patients who do not comply with singlepressure CPAP devices. Some patients perceive that the lower pressureduring exhalation is more comfortable, at least while they are awake. Adifficulty with these devices is deciding how to set the IPAP and EPAPpressures. If the EPAP is too low it may be insufficient to preventobstructions, hence some clinicians may set the EPAP pressure to thepressure titrated during the sleep study. In those patients, the IPAPpressure, and thus the average pressure will be higher that thatrequired to eliminate apneas.

Another form of automatically adjusting CPAP device is the ResMedAUTOSET™ SPIRIT™ device. In this device, the CPAP pressure isautomatically increased or decreased in accordance with indications offlow limitation, such as flow flattening, snore, apnea and hypopnea. SeeU.S. Pat. Nos. 5,704,345; 6,029,665; 6,138,675; and 6,363,933. Anadvantage of an automatically adjusting system is that over time thetreatment pressure required may vary for a particular patient and acorrectly functioning automatic system can obviate the need the patientto return for a subsequent sleep study. These patents also describe amethod and apparatus for distinguishing between so-called “central” andobstructive apneas. The contents of all of the aforesaid patents areincorporated by cross-reference.

Another device for treating certain types of Sleep Disordered Breathingsuch as Cheyne-Stokes Respiration (CSR) is the ResMed AutoCS™ device.Among other things, this device provides a supply of air or breathablegas with a smooth comfortable waveform, sophisticated tracking of thepatient's respiratory phase, and servo-control of patient ventilation.See U.S. Pat. Nos. 6,484,719; 6,532,957; and 6,575,163 (the “AutoVPAP”patents). See also U.S. Pat. No. 6,532,959. The contents of thesepatents are all incorporated by cross-reference.

Some OSA patients find treatment with the above devices uncomfortableand they become non-compliant with the therapy. Other patients such ascardiovascular patients with Congestive Heart Failure, patients with REMHypoventilation, and patients with Respiratory Insufficiency could alsobenefit from a more comfortable and/or effective form of therapy.

3. SUMMARY OF THE INVENTION

In accordance with a first aspect of our invention, there is provided amechanical ventilator with a bi-level waveform and an automaticallyadjusting mean pressure.

In accordance with another aspect of our invention, there is provided amechanical ventilator that automatically adjusts End Expiratory Pressurein accordance with airway patency.

The invention also includes in one form a bi-level CPAP device with anautomatically adjusting IPAP.

Another aspect of our invention is to provide a bilevel CPAP devicewhich adjusts the EPAP in accordance with indications of apnea and theIPAP in accordance with indications of flow limitation.

In accordance with another aspect of the invention there is provided amethod and apparatus for determining when mouth leak is occuring.

In accordance with another aspect of the invention there is provided amechanical ventilator that automatically adjusts End Expiratory Pressurein accordance with leak.

In one form of our invention, the proportion into the overallrespiratory cycle of the patient is continuously determined, theproportion into the overall respiratory cycle being used to scale thetime-length of a pressure-time template, with the pressure delivered tothe patient following the shape of the template but having a swingadapted to patient requirements.

Additional aspects of the invention are described in more detail herein.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a structure of the preferred ventilator apparatus forimplementing the methods of the current invention;

FIGS. 2A-2D illustrate some exemplary waveforms delivered according tovarious embodiments of the invention;

FIG. 3 is a pressure waveform template that is a function of a phasevariable;

FIG. 4 is an inspiratory table of the inspiratory portion (0-0.5) of thewaveform of the graph of FIG. 3;

FIG. 5 is an expiratory table of the expiratory portion (0.5-1) of thewaveform of the graph of FIG. 3;

FIG. 6 is a block diagram of an embodiment of a device according to theinvention;

FIG. 7 is another block diagram illustrating an application of pressureto a ventilator of the invention;

FIGS. 8A and 8B are illustrative functions for controlling swing as afunction of an automatic SDB pressure determination process;

FIG. 9 is a graph illustrating control of median pressure by anautomated SDB pressure detection process; and

FIG. 10 is a graph illustrating mouth leak detection.

5. DETAILED DESCRIPTION 5.1 Hardware

A device in accordance with an embodiment of the invention comprises ablower and blower-controller. The blower can deliver a supply of air atpositive pressure 2-40 cmH₂O, but generally in the range of 4-25 cmH₂Oto a patient interface via an air delivery conduit. The device alsoincludes a flow sensor to measure the flow of air along the conduit, andpressure sensors to measure the pressure of air at the blower outlet. Inone form, the device alternatively includes an additional pressuresensor to detect the pressure in the patient interface.

For example, a preferred embodiment of the invention is illustrated inFIG. 1. The ventilator device may include a servo-controlled blower 2, aflow sensor 4 f, pressure sensor 4 p, a mask 6, and an air deliveryconduit 8 for connection between the blower 2 and the mask 6. Exhaustgas is vented via exhaust 13. Mask flow may be measured by a flowsensor, such as a pneumotachograph and differential pressure transducerto derive a flow signal F(t). Alternatively, the pneumotachograph may bereplaced by a bundle of small tubes aligned in parallel with the flowfrom the blower with the pressure difference measured by thedifferential pressure transducer across the bundle. Mask pressure ispreferably measured at a pressure tap using a pressure transducer toderive a pressure signal P_(mask)(t). The pressure sensor 4 p and flowsensor 4 f have been shown only symbolically in FIG. 1 since it isunderstood that those skilled in the art would understand how to measureflow and pressure. Flow F(t) and pressure P_(mask)(t) signals are sentto a controller or microprocessor 15 to derive a pressure request signalP_(Request)(t). The controller or processor is configured and adapted toperform the methodology described in more detail herein. The controlleror processor may include integrated chips, a memory and/or otherinstruction or data storage medium to implement the control methodology.For example, programmed instructions with the control methodology areeither coded on integrated chips in the memory of the device or loadedas software. As those skilled in the art will recognize, analog devicesmay also be implemented in the control apparatus. The apparatus of FIG.1 includes other sensors, communication interfaces and displays, aservo, etc., functional blocks the details of which are not necessaryfor an understanding of the present invention.

5.2 Synchrony: Triggering, Cycling & Phase Determination

In a device which varies treatment pressure throughout the respiratorycycle of the patient, it is more comfortable for the patient if thedevice varies the pressure in synchrony with patient efforts. Asillustrated in FIG. 6, an overall device 64 constructed in accordancewith the principles of our invention includes an automated therapeutictreatment pressure determination component module 60 which determines atreatment pressure component to treat SDB, and a waveform module 62 orprocess for setting or selecting a waveform (square, rounded, smooth,etc.) for modulating pressure in conjunction with the patient'srespiratory cycle. Typically, the device will include a synchrony ortriggering module 68 (controlled by fuzzy logic, pressure, patienteffort, etc.) for determining states or the phase within the patient'srespiratory cycle. Optionally, a central apnea detector 66 may beincluded.

In delivering pressure, the triggering module monitors the patient tomake a determination of the patient's respiratory phase in order tosynchronise the blower output with the patient's efforts. In one form,the determination is whether the patient is inhaling or exhaling (adiscrete variable). In another form, the determination is of theproportion into the overall respiratory cycle of the patient (a periodicvariable). Determination of phase may be made by some combination ofpressure, flow and effort sensors. For example, in one form, when maskpressure drops below a threshold, the transition to inhalation is takento have occurred and correspondingly when mask pressure rises above athreshold, the transition to exhalation is taken to have occurred. Inanother form, when patient respiratory flow exceeds a threshold,inhalation is taken to have started and when patient respiratory flowfalls below a threshold, exhalation is taken to have started. In anotherform, patient respiratory phase is determined from analysing the shapeof the respiratory flow curve and making a number of determinations asto the extent to which it is early inspiration, middle inspiration, lateinspiration, early exhalation, mid-exhalation and late exhalation. SeeU.S. Pat. Nos. 6,484,719; 6,532,957; and 6,575,163. Once a determinationof phase has been made, the shape of waveform to be delivered can bedecided.

In one embodiment, a phase determination may be based on elapsed time ofthe particular portion of the cycle (inspiration or expiration) relativeto a predicted total time for that particular portion of the cycle. Forexample, the phase within the respiratory cycle is determined as a ratioof the elapsed time to the expected total time. Since the total time forthe current cycle is not yet known, it is predicted by using an averagetime taken from a number of previous breaths, preferably taken from theprevious five breaths of the patient. In the preferred embodiment,irregular breaths are excluded from the average. Thus, breaths of lessthan 150 milliseconds indicative of a cough or hiccup are excluded.Those skilled in the art will understand how to determine such a fivebreath average from a flow signal.

With the determination of a continuous phase and utilizing the waveformmodule 62, smooth pressure changes are then implemented utilizing apressure waveform template for inspiration and expiration that maypreferably be stored in tables or arrays. In general, the tables storescaling factors for a predetermined breath length that when multipliedby a pressure amplitude in sequence will result in a pressure waveformthat is represented by the scaling factors. Versions of these tables areillustrated in FIGS. 4 and 5 based on the smooth waveform of FIG. 3. Asthose skilled in the art will recognize, the tables may be substitutedby mathematical equations rather than relying on a look-up operation.Moreover, although a sinusoidal rise/exponential decay curve is shown,the template may represent any shape of waveform, such as a square waveillustrated by the dashed lines of FIG. 3. Use of the phase variableensures that each scaling factor (SF) of the table is utilized at anappropriate time in the patient's respiratory cycle, thereby compressingor expanding the time-length of the template. In this regard, thedetermined phase variable (elapsed time over total expected time) isused to scale the time-length of the template.

For these ends, a look-up operation that returns a pressure modulatingfunction (Ins_Waveform) utilizing such a phase and an inspiratory tableor array may be accomplished with the following:Ins_Waveform=Inspiratory_Table (I-Location)

where:

-   -   Inspiratory_Table is a function that returns a scaling factor        from the table or array at a particular location determined by        the I-Location function;    -   I-Location is a function that essentially identifies an        inspiration phase variable within the current inspiration (i.e.,        the ratio of the elapsed inspiration time to the expected total        inspiration time) of the patient as this relates to selecting an        appropriate scaling factor from the table that will permit the        delivered pressure to be modulated like that of a normal        inspiration within the time scale of the patient's current        predicted inspiration.

To accomplish this, the function returns an index of the inspiratorytable as a function of (a) the current elapsed time of the inspiration(t_switch), (b) the time-length of the inspiratory portion of thewaveform of the waveform template, which may be the number of table orarray entries over which the inspiratory portion of the waveformtemplate has been recorded in the table (total_entries); and (c) thepredicted time for the inspiration portion of the respiratory cycle,i.e., the average inspiratory time (ave_insp_time). This index may bederived by the following formula:index=round (t_switch*total_entries/ave_insp_time)

Similarly, the look-up operation (Exp_Waveform) on the expiratory tableor array may be accomplished with the following functions:Exp_Waveform=Expiratory_Table (E-Location)

where:

-   -   Expiratory_Table is a function that returns a scaling factor        from the expiration table or array at a particular location        determined by the E-Location function;    -   Much like the I-location function described above, E-Location is        a function that essentially identifies an expiration phase        variable within the current expiration of the patient (i.e., the        ratio of the elapsed expiration time to the expected total        expiration time) for the purpose of selecting an appropriate        scaling factor from the table that will permit the delivered        pressure to be modulated like that of a normal expiration but        within the time scale of the patient's current predicted        expiration.

To accomplish this, the function identifies an index of the expiratorytable as a function of (a) the current elapsed time of the expiration(t_switch), (b) the time-length of the expiratory portion of thewaveform of the waveform template, which may be the number of table orarray entries over which the expiratory portion of the waveform templatehas been recorded in the table (total_entries); and (c) the predictedtime for the inspiration portion of the respiratory cycle, i.e., theaverage inspiratory time (ave_exp_time). The index may be derived by thefollowing formula:index=round (t_switch*total_entries/ave_exp_time)Those skilled in the art will recognize that other formulas may beutilized for modulating the ventilation pressure, such as a more simplecycling and triggering between an IPAP and EPAP.

5.3 Indications of Sleep Disordered Breathing (SDB)

There are a number indications which can be used to detect sleepdisordered breathing including snore, apnea, hypopnea and the shape ofthe inspiratory flow-time curve.

5.3.1 Snore

Snoring can be detected by a number of techniques as known in the art.For example, U.S. Pat. No. 5,245,995 (Sullivan et al.) and U.S. Pat. No.5,704,345 (Berthon-Jones). For example a flow signal can be band-passfiltered in the frequency range of 30-300 Hz and the intensity of theresultant signal determined.

Having detected snoring, a snore index can be calculated as described inthe above patents. The index can be based on the intensity, frequencyand duration of snoring. A “snore prevention pressure” can be defined asthe minimum CPAP pressure necessary to prevent snoring.

5.3.2 Apneas & Hypopneas

Apneas can be detected using a number of techniques as known in the art.For example, U.S. Pat. No. 5,704,345 (Berthon-Jones) describes an apneadetector as follows: The average flow signal variance calculated over amoving time window is compared with a threshold by a level detector, togenerate an “airflow-ceased” trigger. This starts a timer. If thetrigger persists for more than 10 seconds, a comparator declares anapnea. The threshold may be a fixed value, typically 0.1 l/sec, or maybe a chosen percentage (typically 10 or 20%) of the average ventilationover the last several minutes (typically 5 minutes). For convenience,instead of comparing the threshold with the square root of the variance,one can square the threshold, and compare with the variance directly.

Conversely, if airflow resumes before 10 seconds lapses, the timer isreset and no apnea is declared.

In one form, upon detection of an apnea, the device increases pressureuntil the apnea stops, or a predefined threshold is reached. In this waythe pressure does not exceed safe levels. The amount of the increase inpressure can vary upon the pressure at which an apnea has occurred andthe duration of the apnea. See U.S. Pat. No. 6,367,474 (Berthon-Jones etal.)

Hypopneas can be similarly detected, however with a higher threshold,such as 50% of the average ventilation.

5.3.3 Flow Limitation

Flow limitation can be detected by a number of techniques as known inthe art. For example see U.S. Pat. No. 5,704,345 (Berthon-Jones) whichdescribes using various flattening indices such as based on themid-portion of the inspiratory flow-time curve. Other flattening indicesare known. See U.S. Pat. No. 6,814,073 (Wickham). A flow limitationprevention pressure can be defined as the minimum CPAP pressurenecessary to prevent flow limitation. This pressure is notpredetermined, but calculated continuously in conjunction with variousflow flattening indices. Pressure is increased until the indicesindicate that flattening has been eliminated. The pressure level atwhich the indices indicate that flattening is eliminated is taken to bethe current “flow limitation prevention pressure”. In one form of ourinvention, the current “flow limitation prevention pressure” isautomatically decreased with time unless an indication of flowlimitation is detected.

5.4 Waveform Shape

Accordingly, the blower can deliver a generally square-shaped waveform,similar to that provided by the ResMed VPAP™ Series or a more roundedwaveform, similar to that provided by the ResMed AUTOSET CS™ product(see U.S. Pat. No. 6,532,959), for example, having a sinusoidal rise andexponential decay. Furthermore, the blower can adjust the shape of thewaveform between square and more rounded to balance comfort andeffectiveness (See U.S. Pat. No. 6,553,992). In one form, for example,when delivering a generally square wave, the device provides a higherpressure during an inspiratory portion of the patient's respiratorycycle (IPAP) and a lower pressure during an expiratory portion of thepatient's respiratory cycle (EPAP). In another form, for example, whendelivering a more rounded waveform, a SWING and pressure setting aredetermined. The SWING is the difference between the highest and lowestpressures delivered to the patient. The pressure setting could be thebase pressure in one form, or the peak pressure in another form.

5.5 Some Embodiments of our Invention 5.5.1 First Embodiment: AutomaticIPAP

In accordance with a first embodiment of the invention, a device isadapted to provide a generally square waveform having settings for IPAPand EPAP in a manner similar to ResMed's VPAP III device. The device hasa clinician-settable pressure setting for the difference between IPAPand EPAP, ΔP (sometimes called Swing). The device monitors patient flowand determines a treatment pressure using the same algorithm as ResMed'sAUTOSET SPIRIT device. See U.S. Pat. No. 5,704,345. Upon detection ofthe beginning of inspiration, as described above, the device provides asupply of air at the treatment pressure (IPAP=treatment pressure) andmaintains that pressure throughout the inspiratory portion of thepatients breathing cycle. Upon detection of the beginning of exhalation,the device decreases the supplied pressure to the treatment pressureless ΔP.

In this way the advantages of the automatically adjusting CPAP algorithmare brought to a bi-level CPAP device.

5.5.2 Second Embodiment: Automatic EPAP or EEP

A second embodiment of the invention is similar to the first embodiment,except that the End Expiratory Pressure (EEP) or the EPAP pressure isautomatically adjusted, and the IPAP is a fixed delta pressure above theEPAP.

5.5.3 Third Embodiment: Automatic Mean Pressure

A third embodiment of the invention is similar to the first embodiment,except that the mean or median pressures are automatically adjusted, andthere is a fixed delta pressure between IPAP and EPAP.

5.5.4 Fourth Embodiment: Automatic Swing Control

In accordance with another aspect of the invention, the therapeutictreatment pressure is used to control, the pressure difference betweenIPAP and EPAP, or the “swing”. For example, when the device determinesthat the treatment pressure is small, e.g., 5 cmH₂O, then the swing isset to a small value, e.g., 0 or 1 cmH₂O, and when the treatmentpressure is large, e.g., 15 cmH₂O, the swing is set to a larger value,e.g., 3 or 4 cmH₂O. In this way a device in accordance with theinvention can be controlled to ensure that pressure never drops below athreshold, for example, 4 or 5 cmH₂O. Such a form of control isillustrated by the function of FIG. 8A in which the swing is set as afunction of detected therapeutic treatment pressure (APAP TreatmentPressure). An alternative function is illustrated in FIG. 8B in whichthe swing is restricted from falling below a minimum such as 1 cmH₂O.

As illustrated in FIG. 7, if an event of central apnea is detected,swing may be increased or decreased based on several conditions. Forexample, it may be reduced if partial obstruction is not detected.Alternatively, if based on an historic analysis of flow data performedby the device or a classification preset by a clinician, the currentpatient is a hypoventilator (has experienced hypoventilation), the swingmay be increased, for example, by a step up in the swing by apredetermined amount. Alternatively, if based on an historic analysis offlow data performed by the device or a classification preset by aclinician the current patient is a hyperventilator or CV patient, theswing may be decreased, for example, by stepping down the swing by apredetermined amount.

5.5.5 Fifth Embodiment: “Shark-Fin” Waveform

A fifth embodiment of the invention is similar to embodiments one tothree, except that instead of providing a generally square pressure-timewaveform, the waveform has a “shark-fin” shape as indicated in FIGS. 2D,3, 4 & 5.

5.5.6 Sixth Embodiment: Automatic IPAP and EPAP

In accordance with a sixth aspect of the invention, IPAP and EPAP areseparately automatically adjusted. In this form of the invention, twopressure levels are calculated automatically. The first pressure is theminimum pressure necessary to prevent apneas and hypopneas: The secondpressure is the minimum pressure necessary to prevent flow flattening.The EEP or EPAP is set to the first pressure and the IPAP is set to thesecond pressure.

The first pressure is calculated using the apnea and snore detectorsdescribed above. The second pressure is calculated from flow flatteningor roundness indices as described above. The second pressure iscontrolled to be at least equal to the first pressure, preferably 1-2cmH₂O greater than the first pressure. In addition there is apredetermined maximum difference between the first and second pressures.

In one form, absent an indication of apnea or hypopnea the EPAP pressureis decreased. Similarly in one form, absent an indication of flowflattening the IPAP pressure is decreased.

5.5.7 Other Embodiments

The minimum pressure, P₀, delivered to the patient may be automaticallycontrolled in conjunction with the pressure calculated by the automaticalgorithms P_(therapeutic) and an amplitude, A, calculated from thedesired delta pressure between IPAP and EPAP. With regard to thepreferred pressure delivery formula P(t) illustrated above, suchimplementations may be achieved by setting P₀ in alternative ways asfollows: $P_{0} = \left\{ \begin{matrix}P_{therapeutic} \\{P_{therapeutic} - A} \\{P_{therapeutic} - \frac{A}{2}} \\{P_{therapeutic} - {kA}}\end{matrix} \right.$In the first, the therapeutic adjustments generated from the SDBdetection routine discussed above are applied to the end expiratory orbaseline pressure. In the second, therapeutic adjustments are applied tothe peak or end inspiratory pressure. In the third, therapeutic pressureis applied to the mean. Those skilled in the art will recognize thateach of the first three may be derived by the fourth formula by settingadjustment variable K appropriately. In one form, a waveform templatefunction, such as one illustrated in FIG. 3 (either the solid smoothwaveform or the dashed square wave), may be modified for purposes ofapplying the therapeutic adjustments to either the baseline, peak ormedian by adjusting the y-axis of the template to vary from either 0 to1, −1 to 0, or −0.5 to 0.5 respectively. In such an embodiment, P₀ maybe adjusted directly by the therapeutic pressure determination process.

Similarly, as illustrated in the flow chart of FIG. 7, an embodiment ofa bi-level device having IPAP and EPAP, the apparatus may increase theEPAP pressure as a function of apnea duration determined in the apneadetector 74.

Several examples of resulting waveforms that may be generated by adevice in accordance with various embodiments of the invention areillustrated in FIGS. 2A-2D. In FIG. 2A a square wave is generated basedon a fixed swing. The average of the ventilation pressure modulationgradually rises and then falls over time as a result of the automateddetection of a therapeutic pressure that may be applied to the baseline,peak or median as previously discussed.

In FIG. 2B, the therapeutic pressure is applied to the peak pressure orswing of a square wave while the baseline is held fixed resulting in agradually increased and then decreased swing or peak. This may beaccomplished by varying or controlling the peak and/or the swing as afunction of the therapeutic pressure while maintaining the baselinepressure fixed.

In FIG. 2C, the automatically detected therapeutic pressure is appliedto the baseline pressure or the swing of a square wave. This may beaccomplished by varying or controlling the baseline and/or the swing asa function of the therapeutic pressure while maintaining the peakpressure fixed.

In FIG. 2D, the automatic control described above with regard to FIG. 2Ais illustrated with a smoother waveform function. Those skilled in theart will recognize that the control described in FIGS. 2A to 2C can alsobe applied to the smooth waveform function illustrated in FIG. 2D withresults similar to that of FIGS. 2A to 2C respectively.

5.6 Other Aspects 5.6.1 Mouth Leak Detection

In one form, the apparatus includes mouth leak detection apparatus,particularly useful for detection of mouth leak when a nasal mask isused. During such treatment when a patient's mouth is open, typicallyoccurring during exhalation, air delivered to the nasal mask may escapethrough the mouth, through the nose, and out the mouth if the mouth isopened. This is illustrated in the graph of FIG. 10. Thus, the apparatusmay include a module, such as a software module or process, thatreceives a signal indicative of total leak from the mask, as well as asignal indicating whether the patient is inhaling or exhaling.Additionally or alternatively, the software module receives a signalwhich includes markers indicating trigger and cycle time points. Themouth leak detection software module analyzes the total leak signal anddetermines whether or not there is a mouth leak, or that the mouth hasopened resulting when leak increases coincident with exhalation. Suchopenings or mouth leak determinations may be counted and with the adventof a certain number of such events, which may be recorded, an indicatoror warning from the device may inform the user or patient that a fullface mask should be used. Alternatively, as illustrated in FIG. 7, inthe presence of leak, the EPAP pressure or end expiratory pressure maybe reduced to reduce mouth leak or any other detected mask leak. In oneform, the software module integrates the total leak flow duringinhalation to calculate total inhalation leak volume. A similarcalculation is made to determine total leak exhalation volume. If theleak exhalation volume is greater than the leak inhalation volume by athreshold value, then the software module determines that mouth leak ispresent or that the mouth is opening or open during exhalation. Thefollowing equations may be used:LV_(t) = avg_(Xbreaths)(∫_(t = trigger)^(t = cycle)Leak  𝕕t)LV_(e) = avg_(Xbreaths)(∫_(t = cycle)^(t = trigger)Leak  𝕕t)if  LV_(e)⟩LV_(i) + LV_(threshold)thenmouthleakOnce leak flow has been detected, the result may be flagged bygenerating a warning such as an audible or visible alarm, or recordingthe event in a data log.

5.6.2 Advantages of our Invention

The advantages of our invention are many. Many patients find thedelivery of high levels of positive pressure uncomfortable, and this maydisturb sleep quality. The positive pressure delivered by prior artdevices can lead to mask leaks, mouth leaks, upper airway symptoms, airswallowing and difficulty exhaling. A principle of treatment is todeliver the minimal possible effective pressure. An APAP device isdesigned to deliver the minimal possible effective pressure, which mayvary within a night and from night to night. Bilevel devices have beenused to reduce pressure, during exhalation. Apparatus in accordance withan embodiment of the invention combines the features of APAP withbi-level ventilation to reduce mean pressure and increase comfort.

A disadvantage of bi-level ventilators, such as BiPAP, for providingcomfortable therapy in OSA is the rate of pressure change at thetransition from exhalation to inhalation and from inhalation toexhalation. A device in accordance with an embodiment of the inventionutilises a more gradual change in pressure delivery during a breath, andcopies flow rates observed during normal quiet breathing.

5.6.3 Work of Breathing (WOB)

In patients with poor lung compliance, work of breathing is asignificant contributor to energy expenditure. OSA coexists with reducedlung compliance in some disease states, notably congestive heart failure(CHF) where SDB is present in 50-65% of patients. In CHF, CPAP has beendemonstrated to improve heart function through a variety of mechanisms.CPAP is poorly tolerated in this group. A device in accordance with anembodiment of the invention offers advantages of CPAP plus the abilityto decrease WOB. The bi-level waveforms used by a device in accordancewith an embodiment of the invention reduce muscle work required forventilation. A device in accordance with an embodiment of the inventionmay be useful in reducing metabolic rate and hence demands on the leftventricle.

Central apneas are a feature of worsening congestive heart failure andmay require a different Positive Airway Pressure (PAP) therapy, forexample, that provided by the AUTOSET CS™ device discussed above. Adevice in accordance with an embodiment of the invention can monitor thefrequency of central apneas and provide a warning to the patient orclinician that a different form of treatment is indicated.

Regular CPAP may be difficult to use in CHF due to increasedintrathoracic pressure impeding venous return and reducing systolicblood pressure. The bi-level waveforms and peak mean pressures only whennecessary used by a device in accordance with an embodiment of theinvention will lead to a reduction in this side effect of PAP in CHF.

Non-Invasive ventilators are generally set a pressure that normaliseventilation throughout the night. In Rapid Eye Movement (REM) sleep,physiologic changes in respiratory drive, respiratory muscle tone andupper airway tone predispose to added reductions in ventilation comparedto other sleep states. Ventilation required during other sleep statesmay be considerably less. In a device in accordance with an embodimentof the invention, the EPAP pressures are lower during non-REM sleep.

In some patients, particularly those with obesity hypoventilation andother causes of hypercapnic respiratory failure, the increased UpperAirway (UA) resistance encountered in REM is an important factor inobserved reduction in ventilation. In a device in accordance with anembodiment of the invention, EEP is raised in response to airflowflattening in order to counter the effect which a changing UA resistancemay have on ventilation. Thus, in a device in accordance with anembodiment of the invention, by using the minimum necessary EEP requiredto keep upper airway patency, mean pressures are reduced leading toincreased comfort.

Compliance is an issue with low rates of acceptance of therapy with someconditions such as amyotrophic lateral sclerosis and obstructive airwaysdisease. An important factor is expiratory pressure. A device inaccordance with an embodiment of the invention provides for a gradualelevation in expiratory pressures, and lowest possible mean pressuresduring therapy.

Many prior art ventilators use non-physiologic waveforms. While a fastrise-time is important for some patients, for other patients it is moreimportant to have a comfortable waveform. Hence a device in accordancewith an embodiment of the invention provides a range of waveformsincluding more rounded, physiologic shaped waveforms with, for example,a slow falling pressure. The slow falling pressure can be particularlyuseful for Chronic Obstructive Pulmonary Disorder (COPD).

Although the invention has been described with reference to variousembodiments as described above, it is to be understood that theseembodiments are merely illustrative of the application of the variousprinciples of the invention. Numerous modifications, in addition to theillustrative embodiments of the invention discussed herein, may be madeand other arrangements may be devised without departing from the spiritand scope of the invention.

For example other automatic CPAP algorithms may be used to determine thetreatment pressure, such as P_(crit) (for example, as described in U.S.Pat. No. 5,645,053 (Remmers et al.).

1. In a CPAP apparatus having (i) a blower, (ii) a patient interface fora patient, (iii) an air delivery conduit for delivering air from theblower to the patient interface, (iv) a sensor adapted to determine thepressure in the patient interface, (v) a sensor adapted to determine theflow of air to the patient, (vi) a synchrony module programmed todetermine transitions between inspiration and expiration of a patient'sbreathing cycle from at least one sensor and (vii) a control mechanismprogrammed to provide a supply of air at positive pressure in accordancewith a predetermined pressure-time template, a method of controlling theblower operation comprising the steps of: automatically determining atleast one index indicative of the presence of sleep disordered breathingfrom the pressure or flow sensors, automatically determining a treatmentpressure in accordance with the index of sleep disordered breathing,setting at least one characterising parameter of the pressure-timetemplate to the treatment pressure, controlling the blower to deliver asupply of air at positive pressure to the patient in accordance with thetemplate and in synchrony with the patient's breathing cycles asdetermined by the synchrony module.
 2. A method as claimed in claim 1wherein the index indicative of the presence of sleep disorderedbreathing is a function of one or more of flow flattening, snoring,apnea and hypopnea exhibited in the patient's inspiratory flow-timecurve.
 3. A method in accordance with claim 1 in which said template isone of a square wave and a shark-fin wave.
 4. A method in accordancewith claim 1 in which the characterising parameter is a minimum, maximumor mean.
 5. A method in accordance with claim 1 in which thecharacterising parameter is an EPAP or an IPAP.
 6. A method inaccordance with claim 1 in which the characterising parameter is an endexpiratory pressure (EEP).
 7. A method in accordance with claim 1 inwhich the pressure-time template has minimum and maximum pressurevalues.
 8. A method in accordance with claim 1 in which the pressuredelivered to the patient has a minimum value.
 9. A method in accordancewith claim 8 in which the minimum pressure delivered to the patient isabout 4 cmH₂O.
 10. A method in accordance with claim 1 in which thepressure-time template has a fixed swing.
 11. A method in accordancewith claim 1 in which the pressure-time template has a small swing whenthe treatment pressure is low.
 12. A method in accordance with claim 1further comprising the step of determining a second index of sleepdisordered breathing.
 13. A method in accordance with claim 12 whereinthe first index is indicative of apneas and the second index isindicative of flow flattening.
 14. A method in accordance with claim 13further comprising the step of determining a second treatment pressurein accordance with the second index of sleep disordered breathing.
 15. Amethod in accordance with claim 14 further comprising the step ofsetting a second characterising parameter of the pressure-time templateto the second treatment pressure.
 16. A method in accordance with claim15 further comprising the step of setting an EPAP pressure to the firsttreatment pressure and an IPAP pressure to the second treatmentpressure.
 17. A method in accordance with claim 1 wherein saidpressure-time template is stored in look-up tables or arrays.
 18. Inapparatus comprising a blower adapted to provide a supply of air atpositive pressure, a nasal patient interface, an air delivery tubeconnecting the blower to the patient interface, a flow sensor adapted tomonitor the flow of air to a patient along the air delivery tube, and amicroprocessor, a method of detecting the presence of mouth leakcomprising the steps of: determining leak flow during an inspiratoryportion of the respiratory cycle of the patient from the flow sensor;calculating a leak volume during inspiration from the leak flow duringan inspiratory portion; determining leak flow during an expiratoryportion of the respiratory cycle of the patient; calculating a leakvolume during exhalation from the leak flow during the expiratoryportion; flagging that mouth leak has occurred when leak volume duringexhalation exceeds leak volume during inhalation by a threshold.